Localization of earth faults is a challenging task. There are many factors which deteriorate the accuracy of calculated fault location estimates. Distribution networks have some specific features which further complicate and challenge fault localization algorithms. These include e.g. non-homogeneity of lines, presence of laterals and load taps.
One important factor affecting the accuracy of impedance based fault localization algorithms is the combined effect of load current and fault resistance. Also phase-to-earth capacitances in high-impedance earthed systems, especially those of the protected feeder, deteriorate the accuracy of the impedance measurement.
Fault localization algorithms typically utilize either delta quantities (fault state minus healthy state) or symmetrical components to compensate for the effects of load current and fault resistance. In prior art algorithms the load compensation is typically targeted only at current quantities.
Many prior art methods are based on an assumption that load is tapped to the end point of the feeder i.e. load is always located behind the fault point. If this is the case, then the fault location estimate is accurate. Unfortunately, in real medium voltage feeders this assumption is rarely correct. In fact, due to voltage drop considerations, loads are typically located either at the beginning of the feeder or distributed more or less randomly over the entire feeder length. In such cases the accuracy of prior art fault localization algorithms is deteriorated.
Document U.S. Pat. No. 4,313,169 discloses a fault detecting system for locating a fault point. The disclosed solution is based on monitoring variations in voltage and current of a power transmission line resulting from a fault, and calculating the distance to the fault by using the variations and a line constant.
Document U.S. Pat No. 4,996,624 discloses a fault location method for radial transmission and distribution systems. In the method the positive sequence impedance is first determined and then used to determine the distance to the fault.
A problem relating to these solutions is that their application is restricted only to effectively or low-impedance earthed systems. Therefore they cannot be applied in high-impedance earthed networks.